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5 Sheets-Sheet 5 J. B. THOMAS ETAL m NN @NN ELECTROSTATIC PRECIPITATORSNNN Nov. 22, 1960 Filed Aug.. 4, 1959 ELECTROSTATIC PRECIPITATORS JohnB. Thomas, Plainsboro, NJ., and Howard T. Williams and John W. Drenning,Baltimore, Md., assignors to Koppers Company, Inc., a corporation ofDelaware Filed Aug. 4, 1959, Ser. No. 831,514

17 Claims. (Cl. 315-111) This invention relates generally toelectrostatic precipitators and, more particularly, to an apparatus forcontrolling the power supply to the ionizing electrodes.

In conventional electrical precipitators, alternating current from asuitable source is stepped up to a high voltage and rectified to providedirect current. This high voltage direct current is applied to thedischarge electrode and ows to the collector electrode of theprecipitator. As the particles of entrained matter pass the dischargeelectrode, the particles are charged by means of a gaseous dischargeknown as corona. These charged particles are collected principally onthe surface of a collecting electrode. Periodically, the particles whichare collected are removed from the latter electrode by suitable meanssuch as, for example, rapping.

As a result of this gaseous discharge or corona, a current flows throughthe discharge electrodes. The characteristics of the corona depend to agreat extent upon factors such as pressure, temperature, humidity,chemical composition, and particle loading of the gas. A change in oneor more of these factors, therefore, will affect the current flow to theprecipitator. In many instances, these changes cause a current ow thatis in excess of the capacity of the transformers and rectifiers whichstep up the voltage and change the alternating current to directcurrent. To avoid injury to the transformer and rectifier,V most powersupplies have incorporated therein an over-current tripping deviceoperatively connected with a transformer and rectier. Thus, an increasein current llow beyond the rated value of the power supplies willoperate the tripping device and remove the power from the precipitatorbefore damage to the transformer and rectifier can occur. Thereafter,the precipitator remains inoperative until an operator can re-energizethe unit by setting the tripping device manually. To prevent suchexcessive current flow and the resultant inoperativeness of theprecipitator, there has been employed a control means for maintaining aconstant ow of current in the electrical energizing equipment. Suchcontrol means are usually adjusted to deliver maximum possible currentto the precipitator and are known generally as current controls.

In some instances there will occur within the precipitator certainconditions of gas compositions, dust loading, and the like which willcause a complete breakdown of the gas between the precipitatorelectrodes. This breakdown, known as a spark, manifests itself by largesurges of current in the power supply and by momentary loss of thevoltage on the electrodes. No dust collection takes place during themomentary loss of voltage. In practice, the duration of this loss ofvoltage is so short that sparking can be tolerated at rates up to about100 times a minute before the long term collection efficiency of theprecipitator is adversely affected.

It is well known that a precipitator operating at and near the sparkingvoltage so that not more than about 100 sparks per minute occur isoperating at maximum cleaning efficiency. Should theV sparking rateexceed l rates Patenti@ F 2,961,577 Patented Nov. 22, 1960 approximatelysparks per minute, collection efciency is reduced and in many instancesthe current surges in the power supply will cause the overcurrentprotective device to trip resulting in complete loss of cleaningeftciency.

Accordingly, to achieve maximum elciency, the sparking rate mustbemaintained at an optimum even though the internal conditions within theprecipitator may vary. Heretofore, this has been achieved by manuallyregulating the input voltage to the equipment or, alternately, controldevices known as spark rate controls have been incorporated in the powersystem to automatically provide a predetermined sparking rate.

From the foregoing, it is readily apparent that the basic problems incontrolling the power supplies to the precipitator are twofold. Firstly,the current llow to the precipitator must be maintained below some valuecommensurate with the current rating of the transformers and rectifersto prevent-trip-out of the over-current devices from occurring duringload changes; and secondly, the spark rate must be controlled at anoptimum rate commensurate with the precipitator conditions as describedabove.

Heretofore, both of these basic control problems have been met by theuse of either a separate current or separate spark control system. Thisis not entirely satisfactory for primarily two reasons. In most cases,firstly, it is impossible to predict whether the applied voltage will belimited by spark-over or the power supply current rating and, therefore,the type of control required must be determined by trial after theinstallation is operating. Secondly, on those units which normallyoperate from a current control, abnormal sparking may occur when largepatches of dust are dislodged from the collector plates by the rappingsystems. The current control does not readily compensate for suchoccurrences.

An object of the present invention is to provide a novel means fordelivering maximum power to a precipitator incorporating therein a meansfor controlling the current flow and voltage so as to achieve optimumefficiency of the precipitator. 1

. Another object is to provide a novel means for delivering the maximumpossible power to a precipitator during normal and adverse operatingconditions.

A further object is to provide a novel electrical precipitator whereinthe current flow of the electrical energizing equipment is 1continuously sensed to determine the load conditions and corrections aremade in accordance with the load condition sensed to adjust the voltagewhen the need arises to maintain thecurrent flow constant.

In many applications of an electrostatic precipitator,

the characteristics of the precipitator structure such as the clearancebetween the electrodes, the conditions of the gas as, for example,chemical composition and the like, and the dust concentration, may besuch that sparking (also known as Hash-over and spark-over) occurs.Also, frequently even in the absence of prevalent spark-over conditions,spark-over may occur when large-patches of dust are dislodged from theprecipitator plates. .This spark-over is in the nature of a shortcircuit and if it is not rapidly corrected, may cause damage to theprecipitator or the power equipment. In addition, when sparkover occurstoo often, the average voltage on the precipitator issubstantially-reduced thereby reducing cleaning efficiency. Thisspark-over may be controlled by presetting the voltage which is appliedtothe discharge electrode to a low value. However, when the appliedvoltage is preset to a value at which no sparking occurs, the eciency ofthe precipitator .is again materially reduced.

Accordinglyit hasbeen found that in precipitator 'applications where theconditions are such that sparking occurs, maximum precipitatorefiiciency may' be achieved by controlling the applied voltage such thatintermittent sparking occurs. To this end, most efiicient performance isobtained when the voltage between the electrodes of the precipitator israised to a value at which sparks Y'start to jump between theelectrodes.

Still another object is to provide a novel power control unit for aprecipitator wherein there is employed a common current sensing meanswhich effective to energize selectively in accordance with the loadconditions sensed, a system for controlling the voltage so as tomaintain the current substantially constant or to extinguish sparkover,whichever operation is warranted by the load condition sensed.

A still further object of the invention is to provide a novel power unitfor a precipitator wherein the current flow is maintained substantiallyconstant or wherein upon spark-over, the voltage is regulated at apredetermined rate so as to deliver the maximum possible power under theprevailing load conditions.

Yet another object is to provide a spark rate control incorporating anovel means for setting the amount of voltage correction and restoringrate.

The present invention contemplates a novel precipitator for removingforeign particles from a gas wherein provision is made for sensing theliow of electrical current to the electrical energizing equipment andcomparing the sensed signal with a current standard; should thecomparison relative to said current standard reveal an error to bepresent during normal current flow, provision is made for the flow ofcurrent to be adjusted so as to remove the error and maintain thecurrent flow at the standard; should the comparison relative to saidcurrent standard reveal that a spark-over resulting in a surge ofcurrent exists, the current adjusting provision is inoperative andprovision is made to lower the applied voltage and extinguish thespark-over.

The above and further objects aud novel features of the invention willappear more fully from the following detailed description when the sameis read in connection with the accompanying drawings. It is to beexpressly understood, however, that the drawings are not intended to bea definition of the limits of the invention, but are for the purpose ofillustration only.

In the drawings:

Fig. 1 is a schematic block diagram of one embodiment for carrying outthe present invention.

Fig. 2 shows diagrammatically a circuit embodying the inventionillustrated in Fig. l.

Fig. 3 is a graph showing the characteristic saturation curve of theself-saturating magnetic amplifier employed in the electrical energycontrol device.

Fig. 4 is a schematic block diagram of a second ernbodiment for carryingout the invention.

Fig. 5 shows diagrammatically a circuit embodying the inventionillustrated in Fig. 4.

Referring to Figs. 1 and 2, the electrical precipitator is representedschematically as comprising generally a source for supplying alternatingcurrent, a transformer 12 for raising the voltage level of thealternating current, a rectifier 13 for changing the alternating currentto direct current, a discharge electrode 15 for charging the particles,a collecting electrode 17 for collecting the particles, a currentcontrol circuit generally designated as C for maintaining a constantcurrent flow to the discharge electrode, and a spark-rate control systemgenerally designated as V for regulating the voltage in accordance witha predetermined spark rate.

Source 10 for supplying alternating current may be a conventionalalternator. The alternating current is supplied to a current owresponsive device 19, to transformer 12 and after transformation andrectification, to electrode 15.

Transformer 12 and rectifier 13 may be of conventional types. Forexample, mechanical, selenium, silicon, or

vacuum tube types of rectiers may be used. Such transformation andrectification units are normally provided with an over-current device(not shown) for their protection from overload currents.

Discharge electrode 15 is represented as a wire extending axially intotubular collecting electrode 17. As the gas, bearing suspended particlesof matter, passes through electrode 17, the particles are charged byelectrode 15 and deposited principally on the inner surface of electrode17. The material collected on electrode 17 is removed by rapping orirrigation at selected intervals.

As discussed before, the characteristics of the corona may vary withchanges in the gas concentration and composition. These changes maygreatly affect the power input to the precipitator. In accordance withthe present invention, the power input is controlled to obtain maximumefficiency by the current control circuit C which is operative tomaintain the current ow at a preset value in the absence of sparking andspark-rate control circuit V which is operative to regulate the voltageat sparking to periodically extinguish the same and maintain apredetermined sparking rate.

The changes of current fiow caused by normal variation in coronadischarge are reflected in gradual variations in the current flow fromthe preset value. These gradual variations in current are in contrast tothe transient surges of current occurring at spark-over which result insubstantially instantaneous increase in current iiow usually of greatermagnitude than the current ow variation during corona discharge.

The changes in normal current flow and at sparkfover are sensed by thecurrent sensing device 19 which includes a conventional currentresponsive transformer whose primary winding 21 is connected in the lineleading to the discharge electrode 15 and in whose secondary winding 23is developed a voltage corresponding in amplitude to the amount ofcurrent flow through the primary winding 21. By way of potentiometer 25,the amplitude of the voltage is adjusted to a desired level and appliedthrough a coupling transformer 27 to a full wave rectifier 25.

Connected to the rectifier 29 is the current control circuit C and thevoltage control circuit V.

In accordance with the embodiment illustrated in Figs. 1 and 2, thecurrent control circuit C employs the current sensing device 19, areference signal device 44 for developing a signal corresponding to thepreset current ow to the discharge electrode 15, a wave forming device34 for developing a signal corresponding to the actual current flow tothe discharge electrode, a comparator 41 for comparing the referencecurrent signal and the actual current ow signal, an amplifying device 54for developing amplified signal corresponding to the amount by which thecurrent flow exceeds the reference signal at the cornparator, and anelectrical energy control device 20 responsive to the amplified signalfor maintaining the current at said preset value.

The positive output of the rectifier 29 is in the form of a pulsatingvoltage having a basic frequency of twice the supply frequency and apeak amplitude substantially proportional to the peak alternatingcurrent fiowing to the transformer 12. As is well known, the pulsatingdirect current voltages originating at the rectier 29 have analternating current component. For the purpose of forming thealternating current component into a substantially sinusoidal wavepattern, the pulsating current is passed through the wave forming device34, which is an incomplete filter comprising an inductor 35, resistor37, and capacitor 39. The resulting positive sinusoidal alternatingcurrent wave formed by the incomplete filter has an amplitudecorresponding approximately to the root means square value of the loadcurrent at the discharge electrode. Accordingly, any change in the loadcurrent is reflected in the amplitude of the alternating voltage signalpassing through the filter 34.

The alternating voltage signal from the wave forming device 34 isapplied by way of a coupling capacitor'40 to the comparator 41 whichcomprises the amplifier tube 43, the grid resistor 41a resistor 45 andcapacitor 47. The capacitor 40 serves to block out any direct currentvoltage signal and permits only the sinusoidal alternating currentvoltage signal to be applied to the comparator 41.

Connected to the comparator 41 is the reference signal device forapplying a reference voltage signal proportional to the desired currentflow to the discharge electrode 15. The reference signal devicecomprises a po tentiometer 49 which is connected to a suitable source ofnegative direct current. The potentiometer 49 is adjusted to provide aconstant level negative bias on the grid resistor 41a opposite to thepeak amplitude of the pulsating alternating current voltage signalcorresponding to the preset current ow to the precipitator by an amountequal to the cutoff voltage of the tube 43. With this arrangement, novoltage signal is transmitted through the pentode amplifier tube 43until the total algebraic sum of the positive alternating current andthe negative bias signal exceeds the cutoff voltage of the tube. In thisconnection, it is to be noted that no voltage signal or, as will be morefully explained below, no control effect is transmitted through thepentode tube until the amplitude of the positive alternating currentvoltage signal exceeds that corresponding to the preset current ow.

In carrying out the invention, the current flow is preset atsubstantially the maximum current rating of the transformer 12 andrectifier 13. Accordingly, when the sum of constant level bias signaland the positive alternating current voltage at the comparator 41 isless than cutoff voltage of the pentode tube, no voltage signal isemitted therefrom. However, should the sum of these voltages exceed thecutoff value of the pentode tube 43, a pulse signal is emitted from theplate 45. This pulse voltage signal corresponds to the error between thedesired preset current flow and the actual current flow to thetransformer 12.

The voltage pulse error signal emitted from the plate 45 appears at theresistor 51 as an amplified signal, by Way of the amplifying device 54comprising the resistor I53 and the capacitor 55 each of which elementswith the pentode tube 45 form a stage of resistance coupledamplification well known in the art.

The amplified alternating current pulse error signal passes through arectifier 57 which, as shown, comprises a diode tube from the plate 57aof which there is emitted a negative direct current voltagecorresponding to the error between the preset current flow and theactual current ow to the transformer 12.

This negative direct current voltage signal appears by way of a filtercomprising resistor 56 and capacitor 56a at the grid 59 of a triode 61and decreases the current flow through the plate circuit so as to causea decrease in negative current flow to the electrical energy controldevice 20. Since the negative signal introduced on the grid 59corresponds to the error between the preset and actual current fiow, thedecrease in negative current flow, of course, also corresponds to theerror.

The electrical energy control device 20 comprises an amplifying devicein the form of a self-saturating magnetic amplifier 65 and a device forregulating the power input to the precipitator in the form of asaturable reactor 67 connected between the source and the precipitator.

The self-saturating magnetic amplifier 65 includes control windings 69and 71, bias winding 73, and controlled windings 7-5 wound on asaturable core. Windings 75 provide controlled output for the saturablereactor 67 and include saturating rectifiers 77 and 79 connected tosource 10 and direct current output rectiers 81 and 83 connected to thesaturable reactor 67.

The bias Winding 73 is connected in series with a current adjustingrheostat 85 connected to a suitable source of direct current. In theembodiment illustrated, the current flowing through the control winding69 produces negative ampere turns of control and the current fiowingthrough the control winding 71 and the bias winding 73 produces positiveampere turns of control. The ampere turns of control as shown on thecharacteristic saturation curve illustrated in Fig. 3 is the algebraicsum of the ampere turns in the control windings 69 and 71 and the biaswinding 73. When zero current, i.e., zero ampere turns of control are inthe bias winding 73 and the control winding 69, current flowing throughcontrol winding 71 produces positive ampere turns and causes themagnetic amplifier to be fully saturated at the point of the curve ofFig. 3. To achieve the desired control, sufficient current is caused toflow through a bias winding 73 and Winding 71 to add negative ampereturns of control such that during normal operation of the precipitatorthe magnetic amplifier operates on the point B of the curve.

Upon a flow of current in the primary of transformer 12 exceeding thepreset value of current ow, the error signal corresponding to such anincrease manifests itself as a decrease in current in the triode 61.This decrease in current in the winding 69 reflects itself in a decreaseof positive ampere turns such that the negative turns predominate,resulting in a drop in output from the controlled windings 75 and areduction in the current ow in the control winding S7 of the saturablereactor 67. The drop in output from the control winding corresponds tothe error signal. Hence, the impedance of the controlled winding 89 isincreased and a current drop corresponding to the error between thedesired and actual current flow to the precipitator results.

As discussed hereinbefore, the conditions within the precipitator may besuch that spark-over occurs. This spark-over manifests itself in pulsesor surges of transient current flow having a magnitude and a rate ofincrease exceeding that of the normal or preset current flow to thedischarge electrode.

In accordance with the present invention, there is provided the sparkrate control circuit V which, in the embodiment shown in Figs. 1 and 2,is connected to the negative terminal 33 of the rectifier 29 and coupledto the positive terminal 31 by way of the capacitor 100 which alsoserves as a spark detector to beexplained below.

The spark rate control circuit V comprises generally a reference signaldevice 101 for developing a signal corresponding to the normal currentflow through the line t0 the discharge electrode 1'5, the spark detector100 for detecting and sensing the presence of transient surges resultingfrom spark-over at the electrode, a comparator 103 at which thereference signal is compared with the sensed detected transient currentsurges, an amplifier 105 for producing a spark indicating signal ofconstant amplitude, and a spark regulator 107 which is connected to theelectrical energy control device 20 periodically lowering the voltage.

As heretofore explained, the current sensing device comprising thecurrent transformer 2.1, potentiometer 25, transformer 27, and rectifier29 develops a direct current signal voltage corresponding to the currentthrough the primary of the precipitator transformer 12. In the presenceof sparking, current transient surges exceeding the normal current floware generated and these surges manifest themselves in a direct currentvoltage signal of short duration at the positive terminal 31. Asheretofore pointed out, under normal current flow, or corona dischargeconditions, the pulsations of direct current are known to include analternating current value which is approximately proportional to theroot mean square value of the alternating current fiowing to thetransformer. When sparking occurs, the magnitude of the surge will besuch that it greatly exceeds the root mean square value of the normalcurrent flow. Accordingly, the normal current flow and the transientsurges provide a convenient source for determining and controlling thesparking at the electrode. Y l y In accordance with the embodiment ofFigs. 1 andi,

positive alternating current signal voltage from terminal 31 ofrectifier 29 is applied to the comparator 103 which as shown constitutesthe grid resistor of a pentode tube 105. The pentode tube serves as theamplifier, as will be more fully explained below. The positivealternating current signal generated by transient current surges passesthrough the capacitor 100 which also serves to block out the directcurrent values in the pulsating signal.

The reference signal device is connected to the negative terminal 33 ofthe rectifier 29 and comprises the inductor 107, capacitor 109, andresistor 111, which serve to smooth out the negative pulsating signalinto a substantially unidirectional direct current signal from therectifier and are so designed as to have a very large time constant suchthat when sparking occurs and result-ant transient surges appear, noappreciable change of voltage takes place at the instant of the spark.The reference signal device, since it is connected to the negativeterminal of the sensing circuit and since it has a long time constantwhich is not instantaneously responsive to the current surges, applies anegative direct current bias voltage on the resistor grid 103 of thepentode tube 10S such that bias voltage varies directly with the normalcurrent ilow to the transformer 12.

To maintain the voltage control circuit inoperative in the absence ofsparking, the values of the inductor 107, capacitor 109 and resistor 111are selected such that the negative direct current bias applied to thecomparator 103 during normal current flow is opposite to and greaterthan the peak amplitude of the positive alternating current voltagesignal passing through the spark-detector 100 by an amount slightlygreater than the cutoff voltage of the tube 105. In this manner, thegrid of tube 105 is maintained suiciently `biased such that no errorsignal is emitted from the cathode 113 of the pentode 105 and no voltagecontrol function is affected in the absence of sparking.

In the presence of sparking, the transient surge of current associatedwith the sparking results in an alternating current voltage signal atthe terminals 31 and 33 of the rectifier 29 `which greatly exceeds thenormal current ow signal emitted therefrom. This increased transientcurrent signal is applied to the comparator 103 by way of the sparkdetector 100 and reference signal devices 101. However, because of thelong time constant in the reference signal device 101, the signalemitted therefrom and applied to the comparator 103 remainssubstantially the same and, accordingly, gorresponds to the current owto the primary of the transformer prior to the surge of current orspark-over. As above described, this reference signal applies a bias onthe pentode 105 slightly greater than the cutoff voltage of the tube105. The tr-ansient current signal from the terminal 31 passes throughthe capacitor 100 forming the spark detector and is also applied to thecomparator 103. Hence, -upon the occurrence of spark-over, the total sumof the negative bias signal from the reference signal device 101 and thepositive Aalternating current voltage signal corresponding to the ow oftransient current from the spark detector appears at the grit' ofpentode tube 105 and exceeds the cutoff voltage of tue latter so as tocause a pulse signal to be emitted from cathode 113.

The pulse signal emitted from the cathode 113 is of substantiallyconstant `amplitude regardless of the magnitude of the spark of thedischarge electrode 15. This is accomplished by selecting the elementsof the tube circuit in such a manner that upon the occurrence ofspark-over, the :peak voltage on the grid of the pentode tube 105 alwaysequals zero thereby to saturate the tube. The constant amplitude pulseappears Aat the spark regulator 107 by way of the `blocking diode tube115 which serves to prevent the charged pulse from leaving the sparkregulator 107 -by way of thecathode circuit.

As shown in Fig. 2, the spark rate regulator comprises a variableresistor 117, a fixed resistor 119, capacitor 120,

`a xed resistor 121, and variable resistor 123 connected in parallel tothe capacitor 120, and a triode tube 127. The amplitude of the pulseapplied to the grid 125 of the tube 127 is controlled by the resistor119 and the variable resistor 117. Maximum amplitude is obtained by theresistor and lesser amplitude to -a selected desired magnitude byadjustment of the variable resistor 117. Adjustment of the amplitude ofthe pulse determines the magnitude of the voltage drop affected throughthe electrical energy control device.

The parallel connected, fixed and variable 121 and 123 are effective tocontrol the rate at which the electrical energy device is effective togradually increase the voltage as more fully to be explained hereafter.

The voltage signal from the capacitor 120 applied to the grid 125 oftriode tube 127 is in the form of a pulse charge and causes the gridvoltage to rapidly increase such that the tube current is accordinglyrapidly increased a corresponding amount. The rapid increase in platecircuit current in tube 127 is applied to the winding 71 of theself-saturating magnetic amplifier `65 of the electrical energy controldevice 20. Accordingly, since as before described, the winding 71 iswound to provide negative ampere turns of control, the increased voltagecauses additional negative turns of control in the winding 71 which, asshown in Fig. 3, results in a substantially instantaneous reduction inthe output from the control Winding 75 of the magnetic amplifier 65.This decrease in voltage is reflected ina decrease in voltage in thecontrol winding 87 ofthe saturable reactor 67 and, accordingly,increases the impedance in the controlled winding 39 so as to reduce thevoltage at the transformer 12. The magnitude of the drop in Voltage atthe transformer 12 will always be approximately equal for eachspark-over. The amount of voltage reduction is dependent upon theamplitude of the pulse applied to the grid 125 of the tube 253. Asbefore explained the magnitude of the pulse is determined by adjustmentof the resistor 117. Preferably the pulse is adjusted to reduce thevoltage a minimum amount capable of extinguishing spark-over atelectrode 15. After the occurrence of sparking the resistors 121, 123,and capacitor cause the voltage on the grid to dissipate gradually. Therate at which the grid voltage disspates is adjusted at the resistor123. Since the. rate of dissipation of grid voltage controls the rate atwhich the negative turns of control are dissipated, it follows that therate of voltage increase at the saturable reactor 67 is controlled byadjustment of the resistor 123. Hence, the time or rates between sparksis controlled by adjustment of the resistor 123.

From the foregoing, it is apparent that upon the occur rence ofspark-over vat the electrodes the voltage in the transformer 12 isinstantaneously lov/ered an amount determined by the resistance appliedat 117 and the time between sparks or spark-over rate is determined bythe resistance applied at 123.

Referring now to Figs. 4 and 5, there is illustrated a second embodimentfor carrying out the invention and wherein like elements are designatedthe same.

As shown in Fig. 4, the electrical precipitator is representedschematically as comprising generally `a source 10 for supplyingalternating current, a transformer 12 for raising the voltage level ofthe alternating current, a rectiiier 13 for changing the alternatingcurrent to direct current, a discharge electrode 15 for charging theparticles, a collecting electrode 17 for collecting the particles, acurrent sensing device 225 for sensing the current at the dischargeelectrode, an electrical energy control device 220 for controlling thecurrent flow and voltage to the precipitator, a current controllingdevice C for maintaining a constant level of current flow to thedischarge electrode 15, and a spark rate circuit V for regulating thespark rate at the discharge electrode 15.

The alternating current is supplied from a suitable alternating currentsource to the transformer 12 and after suitable rectification by thefull wave rectifier 13, the current is supplied to the electrode 15.

As hereinbefore discussed, the conditions within the precipitator mayvary in respect of the corona discharge to cause a current flow from thesource exceeding the current rating of the transformer 12 and rectifier13 or a spark-over may result in currents exceeding the current ratingof the previous mentioned units. In accordance with the presentembodiment illustrated in Figs. 4 and 5, the flow of the current to thedischarge electrode is maintained constant by the current controlcircuit C' or the voltage is regulated by the spark rate control V.

In accordance with the second embodiment the meassure of the flow ofcurrent to the discharge electrode 15 is obtained as the direct currentvalue at the rectifier 13 rather than the alternating current value asin the embodiment of Figs. 1 and 2. The magnitude of the aver- -agepositive direct current ow through the rectifier circuit issubstantially proportional to the magnitude of the root mean squarecurrent through the secondary winding of the transformer 12. Since theroot mean square secondary current is directly proportional to the rootmean squa-re current ow through the primary winding, the average directcurrent flow through the rectifier 221 substantially corresponds to theroot mean square value of the current flow from the source 10. Hence,the direct current at the rectifier is a substantially direct measureo-f ow to the electrode 15 and also a measure of fiow of alternatingcurrent to transformer 12. Connected to the rectifier 219 is the sensingdevice 225 which is in the form of a resistor 225 and produce a voltagesignal corresponding to the fio-w of current to the discharge electrode15 and of the alternating current flow in the transformer 12.

To maintain the current at the discharge electrode 15 substantiallyconstant, the voltage signal from the sensing device 225 is applied tothe current control device C', which comprises generally a filter forforcing a substantial smooth direct current signal corresponding to theactual current flow to the discharge electrode, a reference currentdevice 239 for developing a signal corresponding to a desired presetcurrent flow to the discharge electrode 15, a comparator for comparingthe reference current signal and the actual current signal, anamplifying device for developing an amplified signal corresponding tothe amount by which the current fiow exceeds the reference signal at thecomparator, and an electrical energy control device responsive to theamplified signal for maintaining the current ow at said preset value.

The sensed voltage signal from the resistor 225 is applied to filter 226comprising an inductor 227, capacitor 229, and resistor 231 which servetosubstantially smooth out the ripple of the pulsating direct currentsignal from the rectifier measure of current. Located in the lineconnecting the filter 226 with the sensing device 225 is a diode tube228 which serves to prevent the resistance at 225 from determinin-g thetime constant at which the currentv signal is introduced into thecurrent control circuit C. In this manner, the signal from the filter226 is representative of and substantially corresponds to the currentflowing the rectifier 13 and accordingly at the ydischarge electrode 15.

The direct current voltage signal from the filter 226 is then applied tothe comparator 233 which as shown is the grid resistor of an amplifiertube 237. Also applied to the comparato-r 235 is the voltage from thereference device 239. The reference device includes a source of negativebiasing voltage 241 which is effective by way of adjustment ofpotentiometer 243 to provide a voltage signal proportional to a presetcurrent flow. The negative bias voltage provided by the reference device239 is set to equal the sum of the signal from the filter 226 and thecutoff voltage of the tube 237. In this manner, during the desired orpreset level of current flow to the discharge electrode 15, thealgebraic sum of the signal at the comparator 235 is such that theamplifier tube' 237 is at or below its cutofvoltage so that no errorsignal is emitted on the plate 245 of the tube during the preset currentow or below. However, should the signals at the comparator 235 show thatthe signal voltage from the current sensing device 225 is greater thanthe preset level such that the algebraic sum of the signal voltage andthe constant negative reference voltage result in a value greater thanthe cutoff voltage on the grid resistor 235, the tube 237 conducts -anamplified voltage error signal which serves to operate the electricalenergy adjusting device 220. The signal emitted at the plate 245corresponds to the difference between the actual current flow and thedesired current flow to the discharge electrode 15.

The electrical energizing device 220, ias in the embodiment illustratedin Figs. l and 2, compris a self-saturating magnetic amplifier 65 and asaturable reactor 67.

The self-saturating magnetic amplifier 65 includes control windings 247land 248, bias winding 249 and control-led windings 75 wound on asaturable core. The windings provide controlled output for the saturablereactor 67 and saturating rectifiers 77 and 79 connected to source 10 atX and output rectifiers 81 and 83 connected to the saturable reactor 67at Y. The bias winding 249 is connected in series with a currentadjusting rheostat 251 connected to a suitable source of direct current.In the embodiment illustrated in Fig. 5, the current flowing through thecontrol windings 247 and 248 produces negative ampere turns of controland the current fiowing through the bias winding 249 produces positiveampere turns of contro-l. The ampere turns of control, as shown on thecurve illustrated in Fig. 3, is the algebraic sum of the ampere turns incontrol windings 69 and 71 and the bias winding 73. The tube 253normally carries a quiescent current which results in producing negativeampere turns of control in the winding 248. The bias winding which iswound to produce positive ampere turns is adjusted so as to cancel thequiescent ampere turns resulting from the current flow through the tube253. In the presence of lan error signal when the tube 237 conducts andemits a signal corresponding to the condition where primary currentexceeds the preset value, additional negative ampere turns of controlare supplied to the winding 247 of magnetic amplifier and the outputcurrent from the control windings 75 is reduced. This reduction inoutput of the winding 69 reects itself in a `decrease in current to thecontrol winding 87 of the saturable reactor 67. Hence, the impedance ofthe controlled winding 89 is increased and a current drop correspondingto the error between the desired and .actual current flow to theprecipitator results. In this manner, the current flow to theprecipitator is maintained substantially constant.

For the purpose of controlling the voltage and the spark rate, there isconnected to the sensing device 221 the spark or voltage circuit V.

As previously discussed, when a spark-over results, the power supply iseffectively short-circuited such that the electrical energy stored inthe precipitator capacitance is discharged. The electrical energy isrecharged on the capacitance fro-m the power supply when the spark-overis extinguished. This recharging results in a surge of current of aduration of one-half of one cycle of a line frequency cycle and thesurges are not necessarily of a constant magnitude and may only beslightly larger than the normal desired flow of current to theprecipitator.`

Heretofore, these recharged pulses of current have been employed toindicate when spark-over occurs. However, because the surges are notalways constant and are occasionally too small to be distinguished fromthe normal current pulses, diiculties heretofore have been encountered.

In accordance with the embodiment of Figs. 4 and 5, the sensing device`225 is arranged so that the discharge of distributed capacitance in thetransformer is employed as a spark indicating signal. As shown in Fig.5, the sensing device employs the circuit of the rectifier 219. When aspark occurs, thus discharging the precipitator and short-circuiting thepower supply, the distributed capacitance in the secondary winding ofthe transformer 12 is also discharged. This discharge current from thesecondary winding passes through the rectifier circuit 219 and reflectsitself in transient current surges which are substantially ten or moretimes the normal current ow. These transient current surges have beenfound to be damped rapid oscillations of about microseconds duration andappear at the sensing resistor 225 as an oscillating voltage signal.

The resulting oscillating voltage signal also appears at the currentcontrol circuit C' but because of the short duration of the surge, thefilter 226 including elements 227, 229, and 231 do not respond thereto.In this manner, the spark signal does not affect the current controlcircuit C but is only applied to the spark rate control circuit V.

The spark rate control circuit C comprises generally a pulse formingdevice 261 for converting the rapidly oscillating spark voltage signalto a unidirectional, nonoscillating signal of somewhat longer duration,a pulse control device 263 for forming a pulse of constant magnitude andduration, and an amplifier 295 for amplifying the constant magnitudepulse, and a spark rate device 267 for regulating the voltage inaccordance with a selected spark rate, and the electrical energy controldevice 226 which is common to both the current control circuit andvoltage control for reducing the voltage at the discharge electrode 15.

The pulse forming device 261 comprises diode 269, capacitor 271 andresistor 273 which are arranged to rectify and lter the oscillatingvoltage signal resulting from transient current surges or spark-over andform a substantially unidirectional pulse signal of somewhat longerduration than the oscillating signal.

This unidirectional signal is applied by way of the coupling capacitor275 to the constant pulse magnitude device 263 which, as shown,constitutes a one shot multivibrator of substantially conventionalcircuitry. As shown, the multivibrator 263 comprises a dual triode tube277 including resistors 279, 281, capacitor 283, resistors 285, 287, and289. The output of the multivibrator 263 is rectangular voltage pulse ofwhich the duration is controlled by the values of the capacitor 283 andresistor 285. These values are selected such that the pulse width isless than one-half the period of the alternator 10.

The rectangular pulse signal emitted from the multivibrator 263 appearsby way of capacitor 291 on the grid 293 of the amplifier 295. Alsoapplied to the grid is a bias signal obtained from a suitable source ofdirect current 297 and resistor 299. This bias signal is of a xed valueand arranged so as to maintain the amplifier 295 below its cutol voltagein the absence of a pulse from the multivibrator 263. Upon theoccurrence of a pulse, iov/ever, the tube 295 emits an amplified signalfrom the cathode 301. The grid resistor 303 is selected to limit gridcurrent when the voltage swings positive upon the occurrence of a pulsesignal indicating spark-over.

The signal emitted from the cathode 301 is then applied to the sparkrate circuit 267 which serves to control the voltage and spark rate andincludes a variable resistor 305, fixed resistor 307, capacitor 309, afixed and variable resistance 310 and 311, respectively, connected inparallel to the capacitor 309, and a triode tube 253. Connected in theline between the amplier 295 and the spark rate circuit 267 is ablocking diode 313 which serves to prevent the pulses applied to thecapacitor 309 from feeding back into the amplifier 295. The amplitude ofthe pulse applied to the grid 317 of the tube 253 is controlled by theresistor 307 and the variable resistor 305. Maximum amplitude isachieved by the fixed resistor 307 and a selected lesser amplitude byway of adjustment of the variable resistor 305. Adjustment of these tworesistors determines the magnitude of the voltage drop elected. Thefixed and variable resistors 310 and 311 are eiective to control therate at which the voltage gradually increases at the transformer 12. Themanner in which the resistors 305 and 307 and resistors 310 and 311 areeffective to control the magnitude and voltage increase respectivelywill be more fully explained below.

The tube 253 is connected to the control winding 248 and is self-biasedby way of the resistor 319 such that the negative ampere controlsprovided by way of the winding 248 are in opposition to the positiveampere turns of control provided by the bias winding 249 of themagnitude amplifier 265. The bias winding, as previously described, issufficient to overcome the negative ampere turns of control provided bythe winding 248 during normal current ow, that is in the absence ofsparking, so that maximum output is obtained from the controlledwindings 75 thereby to assure an optimum voltage in the winding 89 ofthe saturable reactor 67. However, in the presence of sparking, themagnitude of the pulse on the grid 317 of the triode tube 253 is such asto increase the flow of current in the plate circuit 321 whereby thenegative ampere turns of control are increased so that, as shown in Fig.3, the output of the controlled windings 75 is accordingly reducedthereby increasing the irnpedance in the controlled winding S9 of thesaturable reactor 67 as previously explained. The amount of reductionachieved by the saturable reactor 67 is dependent upon the amplitude ofthe pulse applied to the grid 317 of the tube 253. Accordingly, sincethe total values of the resistance from the resistors 305 and 307determines the magnitude of the pulse applied to the grid 253,adjustment of the variable resistor 305 controls the amount of voltagereduction upon the occurrence of spark-over. Preferably the resistor 305is set to result in a voltage drop which does not materially reduceprecipitator efficiency. After the occurrence of sparking the resistors311, 310, and capacitor 315 cause the grid voltage and also the negativeampere turns in the control windings to dissipate gradually whereby agradual decrease in voltage in the controlled winding 89 of thesaturable reactor 67 results. The rate at which the voltage to theprecipitator is increased is determined by the value of the resistancesof resistor 310 and 311 since the latter control the decay rate from theplate of the capacitor 309. By adjustment of the variable resistor 311,it is possible to control the rate at which capacitor 309 discharges andin this manner to control the rate of increase of voltage in thecontrolled windings 89. As described in connection with the embodimentof Figs. l and 2, the rate of increase of voltage determines thesparking rate at the discharge electrode 15. Hence, adjustment of thevariable resistor 310 is effective to control the sparking rate.

What is claimed is:

1. A precipitator for removing foreign particles from a gas comprising adischarge electrode for charging said particles, a collecting electrodefor receiving said charged particles, means for supplying voltage and asubstantially constant level current flow to said discharge electrode,means for sensing said current flow during corona discharge and sparkingat said electrode, and for developing a signal corresponding to saidsensed current ow, means for developing a reference signal correspondingto said constant level current flow during corona discharge, meansresponsive to said sensing means when said current flow during coronadischarge exceeds said reference signal for maintaining said currentflow at said constant level, and means responsive to said sensing meansbeing operative solely upon sparking resulting in transient currentsurges for lowering said voltage to a value at which spark-over isdiminished.

2. A precipitator for removing foreign particles from a gas comprising adischarge electrode for charging said particles, a collecting electrodefor receiving said charged particles, means for supplying voltage and asubstantially constant level current flow to said discharge electrode,means for sensing said current ow during corona discharge and sparkingat said electrode, means responsive 'to said sensing means when saidcurrent flow during corona discharge exceeds said constant level currentoW for maintaining said current flow at said constant level, and meansresponsive to said sensing means being operative solely upon sparkingresulting in transient current surges for reducing said voltage to avalue at which spark-over is diminished and including means foradjusting the magnitude and duration of said voltage reduction.

3. In an electrostatic precipitator a power supply system comprisingmeans for supplying a voltage and a substantially constant level currentflow to said precipitator, means for sensing the ow of current to saidprecipitator, means for controlling the ow of current and voltage to theprecipitator including saturable reactor means, means connected betweensaid sensing means and said saturable reactor means responsive to normalcurrent fiow exceeding said constant level current flow to vary theimpedance of the saturable reactor means so as to reduce the currentflow to said precipitator to the constant level fiow, and meansconnected between said sensing means responsive to transient surges ofcurrent resulting from sparking including means for rendering saidresponsive means inoperative during normal current flow and operativeupon transient surges for varying the impedance of said saturablereactor means to instantaneously lower said voltage.

4. An electrical precipitator system comprising a transformer suppliedfrom an alternating current supply source providing a preset constantlevel of current fiow, rectifying means for the output of saidtransformer, a discharge electrode connected to said rectifying means,an electrical energy control system for controlling the flow of currentand voltage to the discharge electrode comprising means for sensing theow of current to said precipitator, saturable reactor means connectedbetween said source and said transformer, a current control circuitconnected to said sensing means and said saturable reactor includingmeans for developing a signal corresponding to the actual current ilowto said discharge electrode, and means for developing a reference signalcorresponding to the preset constant level current flow, means forgenerating a signal corresponding to the dierence between said actualand preset constant level only when said actual current flow exceedssaid preset level of current iiow so as to vary the impedance of saidsaturable reactor means and reduce said current flow to said dischargeelectrodes to said preset value.

5. An electrical precipitator system comprising a transformer suppliedfrom an alternating current supply source providing a presetsubstantially constant level of current ow, rectifying means for theoutput of said transformer, a discharge electrode connected to saidrectifying means, an electrical energy control system for controllingthe fiow of current and voltage to the discharge electrode comprisingmeans for sensing the flow of current to said precipitator, saturablereactor means connected between said source and said transformer, avoltage control circuit connected to said sensing means and saidsaturable reactor means including means being inoperative during normalcurrent iiow and being operative only upon the occurrence of transientcurrent surges resulting from spark-over for developing a pulse signal,a Voltage regulating circuit including means for selectively adjusting 7the magnitude and the rate of the pulse so as to vary the impedance ofsaid saturable reactor means and reduce said voltage to said dischargeelectrode in accordance with a selected spark rate.

6. The invention as defined in claim 5 in which said signal developingmeans comprises a multivibrator.

7. The invention as defined in claim 5 in which said pulse magnitude anddischarge rate means comprise a variable resistor'and a condensor.

8.,Ihe invention as defined in claim 7 in which said signal developingmeans ,comprises a multivibrator and -saidpulse magnitude and rate meanscomprise a variable resistor and a condensor.

9. An electrical precipitator system comprising a transformer suppliedfrom an alternating current supply source providing a preset constantlevel of current flow, rectifying means for the output of saidtransformer, a discharge electrode connected to said rectifying means,an electrical energy control system for controlling the tlow of currentand voltage to the discharge electrode comprising means for sensing theflow of current to said precipitator, saturable reactor means connectedbetween said source and said transformer, a current control circuitconnected to said sensing means and said saturable reactor includingmeans for developing a signal corresponding to the actual current flowto said discharge electrode, means for developing a reference signalcorresponding to the preset constant level current flow, means forgenerating a signal corresponding to the difference between said actualand preset constant level solely when said actual current ow exceedssaid preset level of current flow so as to vary the impedance of saidsaturable reactor means and reduce said current flow to said dischargeelectrodes to said preset value, a voltage control circuit connected tosaid sensing means and said saturable reactor means including meansbeing inoperative during normal current iiow and being operative onlyupon the occurrence of transient current surges resulting fromspark-over for developing a pulse signal, a voltage regulating circuitincluding means for selectively adjusting the magnitude and the rate ofthe pulse so as to vary the impedance of said saturable reactor meansand reduce said voltage to said discharge electrode in accordance with apredetermined spark rate.

10. The invention as defined in claim 9 in which said saturable reactormeans comprises a first saturable reactor having a control winding and acontrolled winding, and a second saturable reactor having controlledwinding means connected to said control Winding and control windingsarranged to be responsive to said error signal and said pulse signals toincrease the impedance :in said first saturable reactor controlwindings.

l1. The invention as defined in claim l0 in which said second saturablemeans comprises a magnetic amplifier.

l2. The invention as defined in claim 10 in which said signal developingmeans comprises a multivibrator.

13. The invention as defined in claim 10 in which vsaid pulse magnitudeand rate means comprises a variable resistor and a condensor.

14. The invention as defined in claim ll in which said signal developingmeans comprises a multivibrator and said pulse magnitude and rate meanscomprises a Variable resistor and a condensor.

l5. A power control for an electrostatic precipitator having atransformer connected to an alternating current source, a rectifier forthe output of said transformer, a discharge electrode connected to saidrectifier, means connected to said rectitier for sensing the currentfiow to said electrode and developing a signal corresponding to saidcurrent flow, a current control means connected between said sensingmeans and said course responsive to variations in current flow relativeto a reference current flow to maintain said current to said dischargeelectrode constant when said current flow exceeds said reference fiow.

16. A power control for an electrostatic precipitator having atransformer connected to an alternating current source, a rectifier forthe output of said transformer, a discharge electrode connected to saidrectier, means connected to said rectifier for sensing the current ow tosaid electrode, and a voltage control means connected between saidsensing means andsaid source -responsive upon sparking at said electrodeto reduce the voltage at said discharge electrode, said voltage controlmeans including means for forming a square wave of substantiallyconstant amplitude and wave length for each spark.

17. The invention as defined in claim 16 in which said voltage controlmeans includes selectively adjustable References Cited in the file ofthis patent UNITED STATES PATENTS Hildebrand May 7, 1940 Stenitz Sept.24, 1957 Notice of Adverse Decision in Interference In Interference No.`9 H. T. Williams and J. W.

adverse to the patentees W Gazette April 30, 1.963.]

2,443 nvo Drennng,

lvng Patent No. 2,961,577, Electrostatic preciptators,

as rendered Dee. 13, 1962, as to claims J. B. Thomas, nal judgment 1, 9,10 and 13.

